The present invention relates to substrates having a surface-release film of silicone on surface thereof. More particularly, the invention relates to substrates having a surface-release film of silicone having improved releasability stability.
As is well known, surface-release papers or sheets are used for temporarily protecting adhesive surfaces coated, for example, with a pressure-sensitive adhesive from inadvertent sticking, the release paper being peeled off before the adhesive material is put to actual use. The most widely used releasing agents are those formulated with a silicone because of their high effectiveness and reliability with respect to surface-releasing performance as compared with other types of releasing agents.
Several types of silicone, i.e., organopolysiloxane, compositions are used for the a purpose of forming a silicone-based surface-release film including those utilizing the addition reaction between an organopolysiloxane having aliphatically unsaturated hydrocarbon groups, e.g., vinyl groups, bonded to the silicon atoms and an organohydrogenpolysiloxane having hydrogen atoms directly bonded to the silicon atoms, in the presence of a platinum catalyst, and those utilizing the silanolic condensation reaction between silanolic hydroxy groups formed by the hydrolysis of alkoxy groups bonded to the silicon atoms of an organopolysiloxane in the presence of a condensation catalyst.
One of the important problems in the art described above involved in using an organopolysiloxane composition of either type is the instability of the releasability of the thus-formed surface-release film of silicone in the lapse of time. This problem manifests itself in two ways. For example, the peeling resistance of an adhesive sheet from the silicone-coated release paper gradually decreases during the period of time between the formation of the surface film of silicone and when an adhesive sheet is applied and bonded to the silicone-coated surface. The peeling resistance of an adhesive sheet applied and bonded to the silicone-coated release paper also gradually increases, on the other hand, when the adhesive sheet is bonded to the silicone-coated release paper over a prolonged period of time, due to loss of the releasability of the silicone film over the lapse of time. This problem of the releasability instability of the surface-release film of silicone is particularly serious when the pressure-sensitive adhesive is of the type formulated with an acrylic resin or when the coating amount of the silicone release composition is small on the release paper.
This problem due to the instability of the releasability of silicone in the lapse of time is also serious when the surface film of the silicone composition has been cured not to a full extent, for example, due to an unduly low curing temperature or unduly short time taken for curing. Although a silicone composition can be cured to any desired full extent by increasing the curing temperature or by extending the curing time, such a measure is of course not practical with respect to energy saving and productivity of the coating works, as well as with respect to the limitations of the substrate material, such as paper, to withstand the curing conditions. Moreover, the problem can be solved usually only incompletely even by an increase in the curing temperature or extension of the curing time. Accordingly, it is eagerly desired to develop a method or material with which the above-mentioned problems can be solved completely.